An oscilloscope is a type of signal measurement device that is utilized to qualitatively and quantitatively analyze waveforms, typically in the time domain, of electrical signals. It generally provides much more information than a logic analyzer, which merely indicates whether a signal is a logic high ("1") or a logic low ("0"). The oscilloscope displays an analog graphical representation, or trace, of one or more input channels. The display screen has graticule lines defining a graph that are used for quantitatively analyzing the trace. Many oscilloscopes are designed to produce these graphs with 8 graticule divisions situated vertically and 10 graticule divisions situated horizontally. In one typical mode of operation, the vertical axis (y in an x-y coordinate system) of the display screen graph represents voltage amplitude, whereas the horizontal axis represents time. In another typical mode of operation, the vertical and horizontal axes each represent an input channel so that two input channels can be visually compared.
When viewing oscilloscope traces of logic signals, it is desirable to know which of "n" different voltage regions the signal is in at any point in time. For example, if the setup time of a electrical flip-flop latch is 5 nanoseconds (ns) and the hold time is 2 ns, it is desirable to know whether or not the data input signal is a valid logic level during the entire 7 ns of setup and hold time with respect to the input clock signal. As another example, it is also often desirable to know if overshoot or undershoot exceeds some specified voltage threshold. With a conventional oscilloscope trace, this determination can be time consuming and contributes to user fatigue.
One known solution involves marking the screen with one or more horizonal lines at the vertical positions of interest. For example, one could adjust the gain and/or offset of the oscilloscope input channel to place the waveform so that the graticule lines of the display serve as horizontal reference lines. This solution is only acceptable if gain and/or offset adjustments are calibrated, or if some form of a calibration signal exists so that the user may make these adjustments.
Another known solution involves using y-markers, which many oscilloscopes provide, to determine the logic level within an area of interest. Unfortunately, there are usually only two vertical voltage markers available, thereby limiting the number of distinct voltage readings and the number of waveforms that can be evaluated at one time. Additionally, both of the foregoing solutions suffer from a lack of instantaneous recognition, which contributes to operator fatigue.
Still another solution is to display a trace with only distinct levels in a vertical arrangement. A logic analyzer is an extreme example of a device that employs this methodology. In a logic analyzer, a given trace can only be plotted in one of two different vertical positions, signifying whether the signal is above or below a specified threshold. Another example is a model TLS216 oscilloscope that is manufactured by and commercially available from Tektronix, Inc., U.S.A. In this oscilloscope, a trace is plotted at three distinct vertical levels to signify valid high, valid low, or intermediate. Although the devices of the latter two examples give instantaneous recognition, they both suffer from one major drawback: they eliminate a large amount f interesting and useful qualitative data, which is the major reason why someone uses an oscilloscope instead of a logic analyzer in the first place.
Thus, a heretofore unaddressed need exists in the industry for an improved system and method for visually indicating on an oscilloscope display the correspondence between areas of interest on a trace and different logic levels.